Various sorts of systems have been researched and developed up to now for communication systems. As one of these systems there is known the spread spectrum communication system (hereinbelow abbreviated to SS communication system).
By this SS communication system, signals such as data, sound, etc. of narrow frequency band are spectrum-spread into a wide frequency band on the transmitter side by using a pseudo noise code (hereinbelow abbreviated to PN code) to be transmitted and on the receiver side the wide frequency band signals thus obtained are inversely spread into the initial narrow frequency band signals by means of a correlator to be reproduced.
It is known that this SS communication system is resistant to external interference, noise, etc. (these are called interference wave) and that it has an interference excluding power corresponding to a process gain (PG). The process gain used here is given by a following formula; ##EQU1## where the radio frequency bandwidth is the bandwidth of the transmitted spread spectrum signal and the information speed is the data speed at the base band.
As an SS communication device (hereinbelow called simply communication device using such an SS communication system resistant to interference wave there is known e.g. that described in FIG. 7 in U.S. patent application Ser. No. 475,185, (now U.S. Pat. No. 5,048,052.
If interference wave exists in the frequency band of the SS signal in the signal received by a receiver constructed as described above, since spurious noise is superposed on correlation spikes in the output of he correlator (output of the SAW convolver), variations are produced in the correlation spikes and the spurious noise. Here a result of a convolution integration of an SS signal in the received signal and an SS signal of a time-inverted reference signal, as indicated in FIG. 14, is called correlation peak and a result of a convolution integration of interference wave in the received signal and an SS signal of the reference signal is called spurious noise.
In such a state, by the prior art system described above, since only two comparators are used only for shaping the waveform, it is not possible to detect the level of the correlation peak and the level of the spurious noise and the threshold setting level for separating the correlation peak from the spurious noise becomes undetermined, which gives rise to erroneous judgment, as indicated in FIG. 15.
As described above, by the system using two comparators, due to the fact that it is not possible to detect the level of he correlation peak and the level of the spurious noise, worsening in the power of excluding interference wave expressed by the process gain takes place.
As an SS communication receiving device taking such a problematical point into account, another prior art system is disclosed in U.S. patent application Ser. No. 725,530. By this system according to the older application it is intended to improve the data reproducing power by reproducing data, while comparing the correlation peak level obtained by peak-holding and integrating the correlation output with the spurious noise level.
However, even by the system according to the older application, it is necessary to use two convolvers serving as correlators and therefore there is yet room for improvement in the cost and the size reduction.